IF YOU want to believe the rhetoric, Joel Klein is all that stands between America and a vast wave of mega-mergers that will turn the country into one big corporation. As head of the antitrust division of the US Department of Justice, Mr Klein spearheads the nation's competition policy. He is the guiding intelligence behind the campaign to loosen Microsoft's hold on the Internet browser market.

The Microsoft case itself is important enough. But America is currently convulsed by a wave of huge corporate tie-ups, prompting the White House to set up a special team to investigate and appraise the trend. The case against Bill Gates seems to be not just a blow for change in the booming computer software market, but perhaps a straw in the wind for something much bigger. After decades of laissez-faire regulation, is America returning to its older trust-busting ways?

Rhetoric is certainly not Mr Klein's speciality. He is regarded within the legal business as one of the sharpest minds around, but few see him as a crusader. He is described by those who know him as a pragmatist, and though he has known Bill Clinton for many years, few see him in party terms.

Born and raised in the Bronx, Mr Klein, 51, is a product of the elite Harvard Law School. After working as a clerk in the Supreme Court - a highly sought post for young lawyers - he founded his own law firm, before fate brought him to the White House. The suicide of Deputy White House counsel Vince Foster in 1993 took him straight into the legal maelstrom that surrounds President Clinton, advising the First Couple on the Whitewater affair as well as coaching Ruth Bader Ginsburg for her Supreme Court confirmation hearing.

Some are critical, seeing him as a publicity-seeker. "If you want to understand him, don't look at him in ideological terms," says one Washington lawyer. "Look in pure ego terms." Some fellow lawyers see him as a highly political player, who is not unaware of the gains to his department and his career from the Microsoft case. It is fair to say, though, that ego is hardly a rare commodity in Washington; and no one questions Klein's assiduity and intelligence. "He is very analytical, very thoughtful, and very highly though of," says another lawyer.

When he was up for confirmation by the Senate last year, there was widespread suspicion that he was a man who would accommodate himself to big business. The Microsoft case has changed that impression. Suddenly, the campaign against monopoly seems to be on the march again.

Antitrust has played a vital role in shaping modern America, from the destruction of the trusts at the beginning of the century through to the attacks on IBM and AT&T that helped to define the modern telecommunications and computer industries. The anti-monopoly Sherman Act, under which Mr Klein is acting, was passed 105 years ago, part of a wave of populist reaction against the rapid industrialisation after the Civil War. But it was not until the arrival of Thurman Arnold, one of Mr Klein's most distinguished predecessors, in the 1940s, that it really took off. Acting with like-minded judges, Mr Arnold turned it into a powerful tool to break up monopolies, stop price-fixing and prevent agreements that were in restraint of trade.

But from the late 1970s, there was cooling-off. It was no longer fashionable to regard antitrust as a tool of social policy. Instead, more concern was paid to global competition, to the gains that might be realised through mergers and acquisitions, and to free markets.

But despite the latest burst of activism, there is little sign that either Mr Klein or the Justice Department are going back in time. They are trying to work out new approaches, lawyers say. "You're seeing a lot more use of empirical evidence, econometric modelling, and economic analysis," said one lawyer. There is more of a focus on the possible efficiency gains from competition, and Mr Klein (whose first college degree was in economics) has embraced and advanced this.

The Justice Department is also trying to work out how to cope with triple challenges: a vast increase in the amount of merger and acquisition activity, an increase in cross-border activity which requires working with authorities in other countries and a surge in high-technology cases. Mr Klein has indicated his interest in updating laws that were designed for use in traditional industries such as steel and oil to the information age. Nothing could be more important in that context than the World-Wide Web, and the software to access it.

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